The following generally relates to systems and methods for detection of aqueous analytes such as amphiphiles at a liquid crystal interface. The system generally relates to, but is not limited to, liquid crystal—based biosensors, methods of optically detecting liquid crystal birefringence, circular polarization, and the like.
Liquid crystals (LC) are among the first choice of materials for designing devices in the information display technology sector due to their long-range orientational order and fluidity. The long-range orientational order of LCs enable them to transmit ordering at a surface into the bulk of the LC. This ordering in the bulk of the LC results in changes of the LC's effective birefringence, which is an optical property defined by a refractive index that depends on the polarization and propagation of light. This change in effective birefringence is easily detected by optical methods, enabling a quick and easy way of detecting extremely small amounts of surface molecules that have an effect on liquid crystal alignment. Accordingly, LC films may effectively act as sensors that detect when chemical or biological entities are present on an LC surface.
Surfactant-decorated LC surfaces have been developed to detect many biochemical entities and/or associated reactions. This includes the competitive binding of cholic acid (S. He, W. Liang, C. Tanner, K.-L. Cheng, J. Fang, and S.-T. Wu, Anal. Methods 5, 4126 (2013), specific enzymatic reactions (D. Hartono, X. Bi, K.-L. Yang, and L.-Y. L. Yung, Adv. Funct. Mater. 18, 2938 (2008); J. M. Brake, M. K. Daschner, Y.-Y. Luk, and N. L. Abbott, Science 302, 2094 (2003); X. Bi, D. Hartono, and K.-L. Yang, Adv. Funct. Mater. 19, 3760 (2009), bacterial catalase interactions (Q.-Z. Hu and C.-H. Jang, J. Biotechnol. 157, 223 (2012)), heavy metals (Q.-Z. Hu and C.-H. Jang, Colloids Surf. B. Biointerfaces 88, 622 (2011); (S. Yang, C. Wu, H. Tan, Y. Wu, S. Liao, Z. Wu, G. Shen, and R. Yu, Anal. Chem. 85, 14 (2013)), DNA hybridization (A. D. Price and D. K. Schwartz, J. Am. Chem. Soc. 130, 8188 (2008)), and DNA interaction with immobilized oligonucleotides (S. L. Lai, S. Huang, X. Bi, and K.-L. Yang, Langmuir 25, 311 (2009)). 5CB laden with polyacrilic acid block liquid crystalline polymers were used to detect proteins (J.-M. Seo, W. Khan, and S.-Y. Park, Soft Matter 8, 198 (2012)). Specific binding of vesicles was studied at liquid crystal interfaces laden with proteins (L. N. Tan, V. J. Orler, and N. L. Abbott, Langmuir 28, 6364 (2012)). Specific antibody interaction with surface antigen immobilized at LC interface was used for detection of hepatitis B immunocomplex (C.-H. Chen and K.-L. Yang, Anal. Biochem. 421, 321 (2012)).
Surfactants usually promote the alignment of the LC director parallel to the surfactant chains, or homeotropic, and thus on average normal to the substrate. By contrast, water promotes tangential or planar type alignment. The water—LC interface therefore is very sensitive to the presence of surfactants, such as the phospholipids that are found in biological membranes. This is the principle of the LC-based chemical and biological sensing technique introduced by Abbott et al. See R. J. Carlton, J. T. Hunter, D. S. Miller, R. Abbasi, P. C. Mushenheim, L. N. Tan, and N. L. Abbott, Liq. Cryst. Rev. 1, 29 (2013); J. M. Brake, M. K. Daschner, Y.-Y. Luk, and N. L. Abbott, Science 302, 2094 (2003), S. J. Woltman, G. D. Jay, G. P. Crawford, and G. D. J. & G. P. C. Scott J. Woltman, Nat. Mater. 6, 929 (2007).
Previous studies have employed a well-known liquid crystal, 4′-pentyl-4-cyanobiphenyl (5CB) and the homeotropic alignment coating octadecyltrichlorosilane (OTS), although recently a wider range of liquid crystals and alignment coatings have been also tested. See W. Iglesias, N. L. Abbott, E. K. Mann, and A. Jákli, ACS Appl. Mater. Interfaces 4, 6884 (2012). An LC interface with air as the alignment layer also promotes homeotropic anchoring. A biochemical sensor with air replacing the solid homeotropic alignment layer has been previously studied. See D. Hartono, X. Bi, K.-L. Yang, and L.-Y. L. Yung, Adv. Funct. Mater. 18, 2938 (2008). The advantage of using air as a substrate is that air very consistently reproduces homeotropic alignment at any level of humidity, while the quality of alignment at a solid substrate must be carefully controlled.
There is presently a lack of systems, methods, or devices that offer reliable and quantitative detection of amphiphiles at a liquid crystal interface. Present techniques use linear polarizers to analyze the alignment of the liquid crystal, which provides information not only about the birefringence, but also about the tangential distribution of the director. This additional information regarding the director is difficult to control and thus the detection method becomes largely qualitative. Additionally, present detection systems cannot effectively measure both LC film thickness and the effective birefringence of LC materials.
The detection systems, methods, and devices according to the present application include circular polarizers for analyzing the liquid crystal interface. Circular polarizers are sensitive only to birefringence, making the detection much more reliable and quantitative. The detection systems of the present application follow a new procedure that enables not only the concentration dependence of the optical path difference to be determined, but also the film thickness and the effective birefringence to be determined accurately.
In some illustrative aspects disclosed herein, a system for detecting an amphiphile at a liquid crystal interface according to an exemplary embodiment comprises a source of collimated white light, a circular polarizer for polarizing the collimated white light, an LC grid including one or more cells including a suspended nematic LC film, a solution in contact with the LC film at a surface, and a spectrophotometer which optically detects the presence of an amphiphile at the surface by determining a change in birefringence exhibited by the suspended LC film.
In some illustrative aspects disclosed herein, a method for detecting amphiphiles at a liquid crystal water interface comprises shining collimated white light on an LC cell including an LC film, polarizing the white light with a circular polarizer, adding an amphiphile to a solution in contact with the LC film, and optically detecting the presence of the amphiphile by measuring a change in birefringence exhibited by the LC film.